Amplified spontaneous emission reflector-based gain-clamped fiber amplifier

ABSTRACT

Provided is an all-optical gain-clamped fiber amplifier, comprising transmission and isolation means for periodically transmitting an optical signal or reflecting amplified spontaneous emission (ASE) back to a gain medium. The transmission and isolation means can be embodied by an optical interleaver or a number of optical fiber Bragg gratings. Accordingly, an optical signal can be amplified across the entire C-band, and an ASE reflector-based gain-clamped fiber amplifier having a wider dynamic range than conventional amplifiers can be implemented.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2004-79870, filed on Oct. 7, 2004, the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a fiber amplifier that amplifies aninput optical signal in an optical communication system, and morespecifically, to an amplified spontaneous emission (ASE) reflector-basedgain-clamped fiber amplifier that maintain a certain gain characteristiceven when the intensity of the input optical signal varies.

2. Discussion of Related Art

An optical amplifier, such as a semiconductor optical amplifier and afiber amplifier, which is an optical device that amplifies an inputoptical signal, is necessary for compensating for optical loss duringtransmission through optical fiber or various optical devices used inthe field of optical transmission and networking.

However, communication quality in an optical network is not good due tothe drawback that the degree of amplification of the conventionaloptical amplifier varies according to the intensity of the input opticalsignal. Thus, a gain-clamped fiber amplifier has been developed to solvethis problem.

An all-optical gain-clamped fiber amplifier in which a gain is opticallyclamped using a laser cavity does not have a complicated signal processfor clamping the gain. Laser oscillation occurs when a loss and a gaingenerated from the cavity are the same, and once oscillating, amplitudeof population inversion of a gain medium is clamped. The gain of thefiber amplifier is proportional to the amplitude of the populationinversion and a length of the gain medium, so that when the laseroscillation occurs, the gain of the amplifier is clamped. When anoptical signal is input to the fiber amplifier in which the gain isclamped with the laser oscillation, even if the input optical signal isweak, the gain remains constant regardless of the intensity of theoptical signal, and if the intensity of the input optical signal growsstronger, the laser oscillation is paused and the gain-clampedcharacteristic of the fiber amplifier disappears.

FIG. 1 shows an example of the conventional gain-clamped fiber amplifierusing laser oscillation.

An optical fiber is used as a gain medium 1, and a pump light issupplied from a laser diode (LD) 3 through a wavelength divisionmultiplexed (WDM) coupler 2. Using the coupler 4 at input and outputstages, an optical attenuator (ATT) 5, an optical isolator (ISO) 6, anda transmission type optical filter (BPF) 7 constitute a ring cavity. Thetransmission type optical filter 7 adjusts a wavelength at which thelaser oscillation occurs, the optical isolator 6 causes oscillation inthe ring cavity to occur only in one direction, and the opticalattenuator 5 adjusts the gain of the amplifier by controlling opticalloss in the cavity.

In the fiber amplifier shown in FIG. 1, the intensity of the inputoptical signal and the intensity of the laser optical signal generatedtherein have a complementary relationship. In other words, when theintensity of the input optical signal is small, the intensity of thelaser oscillated light is large, and as the intensity of the inputoptical signal grows larger, the intensity of the laser oscillated lightgrows smaller. Thus, even when the intensity of the input optical signalchanges, the amplification ratio remains substantially constant. Here,when the intensity of the input optical signal grows to a certain level,the laser stops oscillating and the gain thereof decreases like a commonfiber amplifier.

However, in the gain-clamped fiber amplifier using the laser cavitydescribed above, when the intensity of the input optical signal varies,the intensity of the optical signal temporarily fluctuates due to arelaxation oscillation in the laser cavity. Such a temporary fluctuationin optical signal intensity has an effect on a bit error rate (BER) oftransmitted data. In addition, a frequency of the relaxation oscillationdepends on the gain medium and the length of the cavity, thereby makingit difficult to fabricate the fiber amplifier and limiting an opticalsignal transmission rate and processing speed.

To solve this problem caused by the laser cavity, the gain-clamped fiberamplifier using an amplified spontaneous emission (ASE) reflector thatcauses ASE to be incident back upon the gain medium, rather than thelaser cavity, has been developed.

The greater the intensity of the optical signal input for amplification,the lesser the intensity of the ASE. Further, the intensity of the ASEreflected by the reflector is proportional to the intensity of the ASE.In other words, when the intensity of the input optical signal is small,the intensity of the ASE becomes so large that the intensity of the ASEreflected by the reflector becomes large, while when the intensity ofthe input optical signal is large, the intensity of the ASE becomes sosmall that the intensity of the ASE reflected by the reflector becomessmall. Consequently, the input optical signal and the reflected ASE areamplified in the gain medium, in which each contributes to the gain.Here, the intensity changes of two beams are opposite to each other, thegain remains almost the same until the intensity of the input opticalsignal grows to a certain level, and when the intensity of the inputoptical signal increases further, the gain of the amplifier is reduced.For convenience, when the clamped gain is lowered by 1 dB, the intensityof the input optical signal is defined as a dynamic range.

An ASE reflector gain-clamped Erbium-doped fiber amplifier (EDFA) usinga reflector in which a mirror is coupled to a 1530 nm CWDM (Coarse WDM)is disclosed in Joon Tae Ahn, et al., “All-Optical Gain-Clamped EDFAWith Improved Noise Figure and Freedom From Relaxation Oscillation”,IEEE Photonics Technology Letters, Vol. 16 No. 1, pp. 84-86, 2004. 1. Inthis amplifier, the stronger the reflected ASE, the wider the dynamicrange. Thus, to obtain a strong reflected ASE, a wavelength of 1531 nm,at which the ASE is the strongest, was included, and 1530 nm CWDM havinga transmission wavelength band of 1521 to 1539 nm was used to reflectthe wavelength as wide as possible. An input optical signal having awavelength of 1550 nm was used and the gain was measured, so that thegain-clamped characteristic could be obtained and the dynamic range wasabout 12 dBm. In addition, with respect to the switching characteristicaccording to the change of the input optical signal, the relaxationoscillation observed in the conventional laser oscillated gain-clampedfiber amplifier was not seen. However, this amplifier has a drawback inthat the 1530 to 1540 nm wavelength portion of the Conventional band(C-band) defined as 1530 to 1565 nm is not amplified due to the 1530 nmCWDM used for the wide dynamic range.

SUMMARY OF THE INVENTION

The present invention is directed to an ASE reflector-based gain-clampedfiber amplifier capable of amplifying optical signals across the entireC-band.

The present invention is also directed to an ASE reflector-basedgain-clamped fiber amplifier with a broader dynamic range than theconventional fiber amplifier.

The present invention is also directed to an ASE reflector-basedgain-clamped fiber amplifier with a lower noise figure factor than theconventional fiber amplifier.

One aspect of the present invention provides an amplified spontaneousemission (ASE) reflector-based gain-clamped fiber amplifier comprising:a gain medium used for amplifying a C-band (1530 to 1565 nm) opticalsignal; a laser diode for supplying pump light to perform populationinversion of the gain medium; a coupler for coupling the pump light tothe gain medium; and transmission and isolation unit for periodicallytransmitting an optical signal provided from the outside to send to thegain medium, and reflecting ASE having a different wavelength from theoptical signal back to the gain medium.

Another aspect of the present invention provides an amplifiedspontaneous emission (ASE) reflector-based gain-clamped fiber amplifiercomprising: a gain medium used for amplifying an optical signal; a laserdiode for supplying pump light to perform population inversion of thegain medium; a coupler for coupling the pump light to the gain medium;and transmission and isolation unit for periodically transmitting theoptical signal amplified from the gain medium to output to the outside,and reflecting ASE having a different wavelength from the optical signalback to the gain medium.

Still another aspect of the present invention provides an amplifiedspontaneous emission (ASE) reflector-based gain-clamped fiber amplifiercomprising: a gain medium used for amplifying an optical signal; a laserdiode for supplying pump light to perform population inversion of thegain medium; a coupler for coupling the pump light to the gain medium;first transmission and isolation unit for periodically transmitting anoptical signal provided from the outside to send to the gain medium, andreflecting ASE having a different wavelength from the optical signalback to the gain medium; and second transmission and isolation unit forperiodically transmitting the optical signal amplified from the gainmedium to output to the outside, and reflecting ASE having a differentwavelength from the optical signal back to the gain medium.

Another aspect of the present invention provides an amplifiedspontaneous emission (ASE) reflector-based gain-clamped fiber amplifiercomprising: a gain medium capable of amplifying a C-band (1530 to 1565nm) optical signal; a laser diode for supplying pump light to performpopulation inversion of the gain medium; a coupler for coupling the pumplight to the gain medium; transmission and isolation unit forperiodically transmitting an optical signal provided from the outside tosend to the gain medium, and reflecting ASE having a differentwavelength from the optical signal back to the gain medium, wherein thetransmission and isolation unit comprises: a reflection mirror; and anoptical interleaver for periodically transmitting the optical signal,isolating the ASE to send to the reflection mirror, and sending the ASEreflected by the reflection mirror to the gain medium; an amplifierlocated between the reflection mirror and the optical interleaver foramplifying the ASE.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent to those of ordinary skill in the art bydescribing in detail exemplary embodiments thereof with reference to theattached drawings in which:

FIG. 1 is a schematic diagram of a conventional gain-clampedErbium-doped fiber amplifier using laser oscillation;

FIG. 2 is a schematic diagram of an amplified spontaneous emission (ASE)reflector-based gain-clamped fiber amplifier according to a firstembodiment of the present invention;

FIG. 3 is a schematic diagram of an amplified spontaneous emission (ASE)reflector-based gain-clamped fiber amplifier according to a secondembodiment of the present invention;

FIGS. 4A to 4C show output spectra of an amplified spontaneous emission(ASE) reflector-based gain-clamped fiber amplifier according to thepresent invention;

FIGS. 5A and 5B are graphs for comparing amplification characteristicsof an amplified spontaneous emission (ASE) reflector-based gain-clampedfiber amplifier according to the present invention and a conventionalfiber amplifier;

FIGS. 6A-6C are graphs for illustrating amplification characteristics ofan amplified spontaneous emission (ASE) reflector-based gain-clampedfiber amplifier according to preferred embodiments of the presentinvention; and

FIG. 7 is a schematic diagram of an amplified spontaneous emission (ASE)reflector-based gain-clamped fiber amplifier according to a thirdembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A gain-clamped fiber amplifier using laser oscillation has a drawback inthat a bit error rate (BER) of a transmitted optical signal is increaseddue to relaxation oscillation of a laser cavity. Meanwhile, an ASEreflector-based gain-clamped fiber amplifier solves the relaxationoscillation problem by not using the laser cavity but has a drawback inthat a signal in the wavelength range of 1530 to 1540 nm of the C-banddefined as 1530 to 1565 nm is not amplified due to 1530 nm CWDM used toobtain a wide dynamic range. Therefore, the present invention isdirected to an ASE reflector-based gain-clamped fiber amplifier capableof expanding an amplified wavelength band and a dynamic range.

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings. The followingembodiments are provided so that those skilled in the art can obtain athorough understanding of the present invention. However, a variety ofmodifications can be made to the described embodiments without departingfrom the spirit and scope of the present invention. Thus, the followingdescription should be considered for explanatory purposes only and notconstrued as limiting the scope of the present invention.

FIG. 2 is a schematic diagram of an amplified spontaneous emission (ASE)reflector-based gain-clamped fiber amplifier according to a firstembodiment of the present invention.

As a gain medium 21 for amplifying an optical signal, for example, anErbium-doped fiber (EDF) is used. A laser diode (LD) 22 provides pumplight having a center wavelength of, for example, 980 nm, for populationinversion of the gain medium 21, and a wavelength division multiplexing(WDM) coupler 23 couples the pump light provided from the laser diode(LD) 22 to the gain medium 21. In addition, a transmission and isolationunit 25 is arranged at an input stage to periodically transmit theoptical signal provided from the outside to send to the gain medium 21through the WDM coupler 23 and send ASE having a different wavelengthfrom the optical signal coming back from the gain medium 21 to areflection mirror 24.

When the pump light provided from the laser diode (LD) 22 is incident onthe gain medium 21 through the WDM coupler 23, each atomic energy levelof the gain medium 21 is changed from a ground state to an excitedstate. An excited atom emits light through stimulated emission, in whichlight having the same wavelength as the optical signal is emitted, andspontaneous emission, in which light having any wavelength is emitted,so that the atom returns to the ground state. Therefore, when the pumplight is incident on the gain medium 21, the optical signal incidentfrom the outside is amplified through the stimulated emission. At thistime, light spontaneously emitted from the gain medium 21 is amplifiedwhile propagating through the gain medium 21, which is referred to asASE. The ASE propagates parallel and anti-parallel to the pump light,and the intensity of the ASE propagating in the opposite direction asthe pump light is typically larger than that propagating in the samedirection as the pump light. Using this property, the ASE propagating inthe opposite direction as the pump light is isolated by the transmissionand isolation unit 25 and directed toward the reflection mirror 24 whereit is reflected to be incident again on the gain medium 21 and amplifiedagain. Here, when the intensity of the input optical signal is low, theASE becomes so strong that the intensity of the ASE reflected from thereflection mirror 24 is large. In contrast, when the intensity of theinput optical signal is large, the ASE becomes weak so that theintensity of the ASE reflected back from the reflection mirror 24 issmall. Therefore, in the gain medium 21, the optical signal is alwaysamplified with a constant gain due to complementary interaction betweenthe input optical signal and the ASE reflected by the reflection mirror24.

FIG. 3 is a schematic diagram of an amplified spontaneous emission (ASE)reflector-based gain-clamped fiber amplifier according to a secondembodiment of the present invention;

As a gain medium 31 for amplifying an optical signal, for example, anErbium-doped fiber (EDF) is used. A laser diode (LD) 32 provides pumplight having a center wavelength of, for example, 980 nm, for populationinversion of the gain medium 31, and a wavelength division multiplexing(WDM) coupler 33 couples the pump light provided from the laser diode(LD) 32 to the gain medium 31. In addition, a transmission and isolationunit 35 is arranged at an output stage to periodically transmit theoptical signal amplified from the gain medium 31 to output to theoutside and send ASE having a different wavelength from the opticalsignal from the gain medium 31 to a reflection mirror 34.

When the pump light provided from the laser diode (LD) 32 is incident onthe gain medium 31 through the WDM coupler 33, each atomic energy levelof the gain medium 31 is changed from a ground state to an excitedstate. The excited atom emits light through stimulated emission, inwhich light having the same wavelength as the optical signal is emitted,and spontaneous emission, in which light having any wavelength isemitted, so that the atom returns to the ground state. Therefore, whenthe pump light is incident on the gain medium 31, the optical signalincident from the outside is amplified through the stimulated emission.Here, light spontaneously emitted from the gain medium 31, i.e., the ASEpropagating in the same direction as the pump light, is isolated by thetransmission and isolation unit 35 and directed toward the reflectionmirror 34 where it is reflected to be incident again on the gain medium31. Therefore, the optical signal is always amplified with a constantgain due to the complementary relationship between the input opticalsignal and the ASE reflected by the reflection mirror 34.

The reflection mirrors 24 and 34 may be made of a typical mirror or anoptical fiber having an end surface coated with dielectric, and thetransmission and isolation units 25 and 35 may comprise, for example, anoptical interleaver. The optical interleaver acts as a multiplexer (MUX)combining two columns of optical signals consisting of optical signalsarranged in periodic wavelengths and deviated with each other by a halfof the wavelength to one column of optical signals having a half periodof the original one. Alternatively, the optical interleaver acts as ademultiplexer (DEMUX) separating one column of optical signals arrangedin periodic wavelengths to two columns of optical signals having a twiceperiod of the original one and deviated with each other by the originalperiod.

A central frequency of the optical signal available in the C-band isdetermined to have an interval of 50 GHz (about 0.4 nm wavelength gap)in accordance with the ITU-T standard. The optical interleaver, such asa 50 GHz interleaver capable of dividing one column of current opticalsignals having a gap of 50 GHz into two columns of optical signalshaving a gap of 100 GHz, or combining two columns of optical signalshaving a gap of 100 GHz into one column of signals having a gap of 50GHz, as well as 100 and 200 interleaver products, are commerciallyavailable.

While Erbium-doped fiber (EDF) is used as a gain medium in FIGS. 2 and3, in addition to Erbium-doped fiber (EDF), a rare earth ion opticalfiber, a fiber Raman optical amplifier, and a semiconductor opticalamplifier (SOA) can each be used as the gain medium. In addition, whileit is advantageous with respect to the noise figure to use laser diodes22 and 32 that generate pump light having a center frequency of 980 nm,laser diodes 22 and 32 that generate a center frequency of 1480 nm canalso be used. Further, an optical isolator that causes light propagatingin one direction of an input stage and an output stage, respectively, tobe transmitted and light propagating in a reverse direction to beabsorbed, can be further arranged.

In addition, as shown in FIG. 3, the reflection mirror 34 and thetransmission and isolation unit 34 can be arranged at the output stageof the fiber amplifier shown in FIG. 2. In this case, the transmissionand isolation unit 25 arranged at the input stage and the transmissionand isolation unit 35 arranged at the output stage should be configuredto reflect the ASE having different wavelengths.

FIGS. 4A to 4C show output spectra of an amplified spontaneous emission(ASE) reflector-based gain-clamped fiber amplifier according to thepresent invention, in which a 50 GHz interleaver is used as thetransmission and isolation units 24 and 35 and an input optical signalhas a wavelength of 1550.12 nm and an intensity of 10 dBm.

FIG. 4A shows the output spectrum of the fiber amplifier shown in FIG.2, FIG. 4B shows the output spectrum of the fiber amplifier shown inFIG. 3, and FIG. 4C is an enlargement of a portion of the spectra shownin FIGS. 4A and 4B.

Since the 50 GHz interleaver was used, the intensity of the ASE wasmodulated with a gap of 100 GHz, i.e., with a wavelength gap of about0.8 nm. The optical signal should be transmitted while reflecting theASE, so it can be appreciated that the amplified optical signal islocated at a trough of the modulated ASE for the amplifier of FIG. 2 andat a ridge of the modulated ASE for the amplifier of FIG. 3. Therefore,with the optical interleaver having an appropriate wavelength gapcharacteristic that allows the optical signals intended foramplification to be transmitted, the optical signals can be amplifiedacross the entire C-band. In addition, the ASE can be reflected across awavelength range corresponding to half of the C-band, as a whole, inaddition to around 1531 nm where the ASE of the Erbium-doped fiber (EDF)is the strongest, so that the intensity of the reflected ASE is greaterthan in the conventional amplifiers. In addition, the fiber amplifier ofthe present invention has a wider dynamic range than the conventionalfiber amplifier.

FIGS. 5A and 5B are graphs of gain and noise figure measurements versusintensity of input optical signal for two cases where an optical signalhaving a wavelength of 1550.12 nm is input to the ASE reflector-basedgain-clamped fiber amplifier according to the embodiments of the presentinvention shown in FIGS. 2 and 3, and to the conventional Erbium-dopedfiber amplifier without gain clamp.

Referring to FIG. 5A, the gain-clamped characteristic, meaning that thegain does not change even when the intensity of the input optical signalvaries, is apparent for the amplifiers of FIG. 2 (circles) and FIG. 3(triangles). Both cases have almost the same clamped gain of 19 dB anddynamic range of about 7 dBm, which constitutes an improvement ofseveral dB compared to the conventional amplifier, as expected.

Referring to FIG. 5B, the noise figure for the amplifier of FIG. 2(circles) is better than that for the amplifier of FIG. 3 (triangles) inthe dynamic range, but both are worse than for the amplifier withoutclamped gain.

FIGS. 6A and 6B are graphs illustrating amplification characteristics ofan ASE reflector-based gain-clamped fiber amplifier according to thepresent invention. Clamped gain is plotted in FIG. 6A, noise figure isplotted in FIG. 6B, and dynamic range is plotted in FIG. 6C, for theamplifiers of FIG. 2 (rectangles) and FIG. 3 (circles), versuswavelength of an input optical signal having an intensity of about 22dBm.

Clamped gains for the amplifiers of FIGS. 2 and 3 are similar across theentire range of wavelengths. In contrast, the noise figure of theamplifier of FIG. 2 is smaller than that of the amplifier of FIG. 3 byabout 0.7 to 1.5 dB, and the dynamic range of the amplifier of FIG. 3 islarger than that of the amplifier of FIG. 2 by about 1 dB.

While in the described embodiments of the present invention, an opticalinterleaver having a periodic transmission and isolation characteristicis used as an ASE reflector-based gain-clamped fiber amplifier capableof amplifying optical signals across the entire C-band, a number offiber Bragg gratings (FBG) may be used instead to reflect the ASE. Inthis case, since the optical signal should be transmitted whilereflecting the ASE, the fiber Bragg gratings (FBG) must have a centerwavelength that is different from a wavelength of the optical signal. Inaddition, as the intensity of the reflected ASE increases, the dynamicrange of the amplifier increases. Thus, it has the advantage ofincreased dynamic range when the reflection wavelengths of a number offiber Bragg gratings (FBG) are around 1531 nm where the intensity of theASE is large. In contrast to the optical interleaver having a periodicwavelength characteristic, the reflection wavelengths of the fiber Bragggratings (FBG) need not be periodic. In the amplifiers of FIGS. 2 and 3,a number of fiber Bragg gratings (FBG) can be used rather than thetransmission and isolation units 25 and 35 and the reflection mirrors 24and 34 used for transmitting the optical signals while selectivelyreflecting the ASE.

FIG. 7 is a schematic diagram of an ASE reflector-based gain-clampedfiber amplifier according to a third embodiment of the presentinvention, in which an amplifier for amplifying the ASE is furtherincluded to increase a dynamic range.

As a gain medium 71 for amplifying an optical signal, for example, anErbium-doped fiber (EDF) is used. A laser diode (LD) 72 provides pumplight having a center wavelength of, for example, 980 nm, for populationinversion of the gain medium 71, and a wavelength division multiplexing(WDM) coupler 73 couples the pump light provided from the laser diode(LD) 72 to the gain medium 71. In addition, a transmission and isolationunits 75 is arranged at an input stage to periodically transmit theoptical signal provided from the outside to send to the gain medium 71through the WDM coupler 73 and send ASE having a different wavelengthfrom the optical signal coming back from the gain medium 71 to areflection mirror 74. An optical amplifier 76 is arranged between thetransmission and isolation units 75 and the reflection mirror 74.

The reflection mirror 74 may be made of a typical mirror or an opticalfiber having an end surface coated with dielectric, and the transmissionand isolation units 75 may comprise, for example, an optical interleaveror an optical filter such as a CWDM.

When the pump light provided from the laser diode (LD) 72 is incident onthe gain medium 71 through the WDM coupler 73, each atomic energy levelof the gain medium 71 is changed from a ground state to an excitedstate. An excited atom emits light through stimulated emission, in whichlight having the same wavelength as the optical signal is emitted, andspontaneous emission, in which light having any wavelength is emitted,so that the atom returns to the ground state. Therefore, when the pumplight is incident on the gain medium 71, the optical signal incidentfrom the outside is amplified through the stimulated emission. At thistime, light spontaneously emitted from the gain medium 71, i.e., the ASEpropagating in the opposite direction to the pump light, is isolated bythe transmission and isolation units 75 and directed toward thereflection mirror 74 where it is reflected back toward the gain medium71 to be amplified. Here, the optical amplifier 76 amplifies the ASEreflected by the reflection mirror 74, so that a complementaryinteraction between the input optical signal and the reflected ASE inthe gain medium 71 is effectively increased. Thus, the gain of theoptical signal can be maintained constant more effectively. In otherwords, since the ASE is amplified before and after being reflected fromthe reflection mirror 74, the intensity of the ASE incident on the gainmedium 71 is significantly increased. Therefore, the dynamic range ofthe fiber amplifier is increased in proportional to the intensity of thereflect ASE reflected incident again on the gain medium 71, so that thedynamic range can be significantly increased according to the presentembodiment. Further, as the intensity of the ASE increases, thepopulation inversion of the gain medium 71 decreases so that the noisefigure increases compared to an optical amplifier that does not employASE amplification.

Further, the third embodiment of the present invention shown in FIG. 7can be applied to the fiber amplifier shown in FIG. 3. In other words,the same effect can be obtained with the optical amplifier 76 of FIG. 7arranged between the transmission and isolation unit 35 and thereflection mirror 34 of the fiber amplifier shown in FIG. 3.

In addition, to increase the characteristic of the noise figure of thepresent invention, an ASE reflector-based gain-clamped fiber amplifieris provided further including a preamplifier for amplifying an inputoptical signal at an input stage of the fiber amplifier shown in FIGS.2, 3, and 7.

Referring to FIG. 5B, it will be appreciated that the noise figure forthe fiber amplifier shown in FIGS. 2 and 3 is larger than that for theamplifier without the clamped gain. In addition, for the fiber amplifiershown in FIG. 7, it will be appreciated that the ASE having a largeintensity is incident on the gain medium by the ASE amplifier arrangedbetween the transmission and isolation unit 35 and the reflection mirror34, so that a dynamic range can be expanded. However, the populationinversion at the input stage of the gain medium is lowered so that thenoise figured grows larger than in the embodiments of FIGS. 2 and 3.Therefore, when the amplifier with which population inversion is highand constant gain can be obtained irrespective of the intensity of theinput optical signal is added to the input stage of the fiber amplifieraccording to the present invention, the gain-clamped characteristic canbe maintained while reducing the noise figure. This can be done bysimply arranging the preamplifier at the input stage of the fiberamplifier shown in any of FIGS. 2, 3, and 7.

As described above, the present invention includes transmission andisolation means for periodically transmitting an optical signal orreflecting amplified spontaneous emission (ASE) to send to a gainmedium. By using an optical interleaver or a number of fiber Bragggratings as the transmission and isolation means, the optical signal canbe amplified across the entire C-band and a dynamic range can beincreased beyond that of conventional fiber amplifiers. In addition,with a preamplifier arranged at an input stage of the ASEreflector-based gain-clamped fiber amplifier as described above, a noisefigure can be effectively reduced.

Exemplary embodiments of the present invention have been described withreference to the attached drawings. However those skilled in the artwill appreciate that various modifications can be made to the describedembodiments without departing from the spirit of the invention. Forexample, the location and type of the laser diode (LD), the length of anErbium-doped fiber, and the location of an optical isolator can beadjusted. In addition, the principle of the present invention can beapplied to other rare earth element-doped optical amplifiers, i.e.,thulium doped fiber or halide oxides active fiber can also be applied.Therefore, the scope of the present invention should be determined bythe appended claims and their equivalents.

1. An amplified spontaneous emission (ASE) reflector-based gain-clampedfiber amplifier, comprising: a gain medium for amplifying an inputoptical signal at a clamped gain; a laser diode for supplying pump lightto perform population inversion of the gain medium such that the opticalsignal is amplified through stimulated emission, and such that ASEpropagating in the same direction as the pump light and having adifferent wavelength from the optical signal is emitted from the gainmedium; a coupler for coupling the pump light to the gain medium; and atransmission and isolation unit configured so as to allow amplificationacross an entire C-band range, the transmission and isolation unit beingconfigured to periodically transmit the amplified optical signal fromthe gain medium to output to the outside, isolate ASE propagating in thedirection of pump light and having a different wavelength from theoptical signal and reflect the ASE propagating in the direction of thepump light so that it propagates back to the gain medium and contributesto the gain.
 2. The amplified spontaneous emission (ASE) reflector-basedgain-clamped fiber amplifier according to claim 1, wherein thetransmission and isolation unit comprises: a reflection mirror; and anoptical interleaver for periodically transmitting the optical signal,isolating the ASE to send to the reflection mirror, and sending the ASEreflected by the reflection mirror to the gain medium.
 3. An amplifiedspontaneous emission (ASE) reflector-based gain-clamped fiber amplifier,comprising: a gain medium used for amplifying an optical signal; a laserdiode for supplying pump light to perform population inversion of thegain medium; a coupler for coupling the pump light to the gain medium;and transmission and isolation unit for periodically transmitting theoptical signal amplified from the gain medium to output to the outside,and reflecting ASE having a different wavelength from the optical signalback to the gain medium, wherein the transmission and isolation unitcomprises: a reflection mirror; and an optical interleaver forperiodically transmitting the optical signal, isolating the ASE to sendto the reflection mirror, and sending the ASE reflected by thereflection mirror to the gain medium; and an amplifier located betweenthe reflection mirror and the optical interleaver for amplifying theASE.
 4. The amplified spontaneous emission (ASE) reflector-basedgain-clamped fiber amplifier according to claim 1, wherein thetransmission and isolation unit comprises a number of optical fiberBragg gratings.
 5. The amplified spontaneous emission (ASE)reflector-based gain-clamped fiber amplifier according to claim 1,wherein the gain medium is made of an Erbium-doped optical fiber, a rareearth ion-doped optical fiber, an optical fiber Raman amplifier, or asemiconductor optical amplifier.
 6. An amplified spontaneous emission(ASE) reflector-based gain-clamped fiber amplifier, comprising: a gainmedium used for amplifying an optical signal; a laser diode forsupplying pump light to perform population inversion of the gain medium;a coupler for coupling the pump light to the gain medium; transmissionand isolation unit for periodically transmitting the optical signalamplified from the gain medium to output to the outside, and reflectingASE having a different wavelength from the optical signal back to thegain medium; and an amplifier arranged at an input stage for amplifyingthe optical signal provided from the outside.
 7. An amplifiedspontaneous emission (ASE) reflector-based gain-clamped fiber amplifier,comprising: a gain medium used for amplifying an optical signal at aclamped gain; a laser diode for supplying pump light to performpopulation inversion of the gain medium; a coupler for coupling the pumplight to the gain medium; a first transmission and isolation unit forperiodically transmitting an optical signal provided from the outside tosend to the gain medium, and reflecting ASE having a differentwavelength from the optical signal back to the gain medium; and a secondtransmission and isolation unit for periodically transmitting theoptical signal amplified from the gain medium to output to the outside,and reflecting ASE having a different wavelength from the optical signalback to the gain medium wherein the first and second transmission andisolation units are configured such that the ASE reflected by the firsttransmission and isolation unit has a different wavelength than the ASEreflected by the second transmission and isolation unit, and the ASEreflected by each of the first and second transmission and isolationunit contributes to the gain.
 8. The amplified spontaneous emission(ASE) reflector-based gain-clamped fiber amplifier according to claim 7,wherein each of the first and second transmission and isolation unitscomprises: a reflection mirror; and an optical interleaver forperiodically transmitting the optical signal, isolating the ASE to sendto the reflection mirror, and sending the ASE reflected by thereflection mirror to the gain medium.
 9. An amplified spontaneousemission (ASE) reflector-based gain-clamped fiber amplifier, comprising:a gain medium used for amplifying an optical signal at a clamped gain; alaser diode for supplying pump light to perform population inversion ofthe gain medium; a coupler for coupling the pump light to the gainmedium; first transmission and isolation unit for periodicallytransmitting an optical signal provided from the outside to send to thegain medium, and reflecting ASE having a different wavelength from theoptical signal back to the gain medium; and second transmission andisolation unit for periodically transmitting the optical signalamplified from the gain medium to output to the outside, and reflectingASE having a different wavelength from the optical signal back to thegain medium, wherein each of the first and second transmission andisolation units comprises: a reflection mirror; and an opticalinterleaver for periodically transmitting the optical signal, isolatingthe ASE to send to the reflection mirror, and sending the ASE reflectedby the reflection mirror to the gain medium; and an amplifier locatedbetween the reflection mirror and the optical interleaver for amplifyingthe ASE.
 10. The amplified spontaneous emission (ASE) reflector-basedgain-clamped fiber amplifier according to claim 7, wherein each of thefirst and second transmission and isolation unit comprises a number ofoptical fiber Bragg gratings.
 11. The amplified spontaneous emission(ASE) reflector-based gain-clamped fiber amplifier according to claim 7,wherein the gain medium is made of an Erbium-doped optical fiber, a rareearth ion-doped optical fiber, an optical fiber Raman amplifier, or asemiconductor optical amplifier.
 12. An amplified spontaneous emission(ASE) reflector-based gain-clamped fiber amplifier, comprising: a gainmedium used for amplifying an optical signal at a clamped gain; a laserdiode for supplying pump light to perform population inversion of thegain medium; a coupler for coupling the pump light to the gain medium; afirst transmission and isolation unit for periodically transmitting anoptical signal provided from the outside to send to the gain medium, andreflecting ASE having a different wavelength from the optical signalback to the gain medium; a second transmission and isolation unit forperiodically transmitting the optical signal amplified from the gainmedium to output to the outside, and reflecting ASE having a differentwavelength from the optical signal back to the gain medium; and anamplifier arranged at an input stage for amplifying the optical signalprovided from the outside.